The present invention is directed generally to the modification of the surface of fluoropolymer sheet materials by a reactive gas plasma process. In another aspect the present invention relates to a method to treat the surface of fluoropolymers with a reactive gas plasma to improve the bonding of fluoropolymers to other substrates without the use of adhesives. In another aspect the present invention relates to a method to treat the surface of fluoropolymers with a reactive gas plasma to improve the bonding of fluoropolymers to metals with the use of adhesives.
There is a need to modify fluoropolymer surfaces so that there is good adhesive bonding of the surface of a fluoropolymer substrate such as film or sheet to substrates such as metals, plastics including fiber reinforced plastics and ceramics. Fluoropolymers are inert, therefore, there is a need to discover processes which bond the surface of a fluoropolymer sheets or films to other substrates. Bonding of fluoropolymer sheet material to other substrates has been accomplished by many different approaches.
The major commercial method of promoting the adhesion of fluoropolymer surfaces to metal surfaces is the alkali metal treatment, wherein sodium and/or potassium metal is reacted with naphthalene/THF or liquid ammonia to produce the treatment salution, see W. M. Riggs & D. W. Dwight, J. Electron Spectros Rel. Phen., 5, 447 (1974), and C. V. Cagle, Adhesive Bonding Techniques and Applications, McGraw Hill, New York. The disadvantages of this approach are that there is a low level of defluorination achieved, the surface degrades easily and there are waste disposal problems.
Another approach for modifying the fluoropolymer surface is the ion bombardment process whereby the surface to be bonded is exposed to a flux of particles of radiation whereby the ions induce a physical change and thereby modify the surface. The radiation roughens the fluoropolymer surface allowing for adherence to the metal surface. The disadvantages of these approaches are that there is shallow depth of treatment, partial defluorination and the creation of unsaturated sites.
Reactive gases plasmas, of hydrogen, oxygen, helium-oxygen, and nitrogen plasmas have been used to treat the surfaces of polypropylene, see D. T. Clark and R. Wilson, J. Polymer Science, Polymer Chem. Edition, 2, 837 (1983), and D. S. Everhart and C. N. Reilley, Surf. Interface Anal., 3, 126 (1981). In accordance with this invention, it has been discovered that the treatment of fluoropolymer surfaces with reactive gas plasma of water, volatile non-polymerizing alcohols and volatile non-polymerizing organic acids renders the fluoropolymer surface reactive, with excellent adhesive bonding properties and long term stability against the degradation of environmental factors such as ultraviolet light, water, air, sunlight and the like.
The use of the reactive plasma gas surface modification of fluoropolymers of this invention allows for uniform modification of the fluoropolymer surface, elimination of waste disposal, simplicity of approach, long term stability of the modified fluoropolymer and excellent fluoropolymer to substrate adhesive bond strength.
It is an object of this invention to provide a method to surface treat fluoropolymer substrates with a reactive gas plasma so as to improve and provide excellent adhesive bonds of the fluoropolymer to other substrates.
It is another object of this invention to prepare fluoropolymer composites without the use of adhesives.
It is another object of this invention to provide a reactive gas plasma surface modification of fluoropolymer process wherein the fluoropolymer surface is reactive to forming strong adhesive bonds to metal and the fluoropolymer surface is stabilized toward environmental degradation of the surface of the fluoropolymer.
These and other objects, together with the advantages over known methods shall become apparent from the specification that follows and are accomplished by the invention as hereinafter described and claimed.